Electrostatic charging and discharging apparatus

ABSTRACT

THIS INVENTION RELATES TO AN APPARATUS FOR THE ELECTROSTATIC CHARGING OR DISCHARGING OF THE SURFACE OF WEB OR SHEET-LIKE MATERIAL, ESPECIALLY THE ELECTROSTATIC CHARGING OF PHOTOCONDUCTIVE INSULATING LAYERS, SAID APPARATUS INCLUDING A CHARGE-ABSORBING ELECTRODE IN THE FORM OF A ROTATABLE ROLLER MEANS WITH A ROUGHENED SURFACE, SAID ROLLER MEANS BEING PARTIALLY SURROUNDED BY AN INSULATOR AND LOCATED BETWEEN POINTED CORONA ELECTRODES IN THE FORM OF A SERIES OF CLOSELY SPACED NEEDLES WHICH ARE EXTERNALLY PROTECTED BY A SECOND INSULATOR, THE OUTER EXTERMITY OF THE SAID ROLLER MEANS BEING SPACED SUBSTANTIALLY EQUALLY TO THE POINTS OF THE NEEDLES FROM THE PATH OF A MATERIAL TO BE PROCESSED BY THE APPARATUS.

G. MARX Feb. 1 1973 ELECTROSTATIC CHARGING AND mscmmcme APPARATUS Filed Dec. 28, 1971 INVENTOR GERHARD MARX ATTORNEY United States ABSTRACT OF THE DESCLUSURE This invention relates to an apparatus for the electrostatic charging or discharging of the surface of web or sheet-like material, especially the electrostatic charging of photoconductive insulating layers, said apparatus including a charge-absorbing electrode in the form of a rotatable roller means with a roughened surface, said roller means being partially surrounded by an insulator and located between pointed corona electrodes in the form of a series of closely spaced needles which are externally protected by a second insulator, the outer extremity of the said roller means being spaced substantially equally to the points of the needles from the path of a material to be processed by the apparatus.

This invention concerns an apparatus for the electrostatic charging or discharging of the surface of web or sheet-like material. 1

Devices by means of which surfaces of materials with dielectric properties can be electrostatically charged or discharged are known. They usually consist of assemblies which emit discharge currents and comprise thin high-ten-- sion wires extending over the entire width of the web.

Other known arrangements comprise, instead of wires, point electrodes which offer advantages due to their more pronounced surface curvature. -It has been found, however, that these assemblies do not satisfy growing technical demands. Corona wires supply only nonuniform discharge currents and, in operation, tend to produce oscillations of considerable amplitude. With high loads, a burned residue forms on point electrodes after a relatively short period of operation and this residue prevents the discharge currents emitted by the electrodes from propagating in the required uniform manner.

In another known arrangement for electrostatic charging, point electrodes in conjunction with laterally arranged auxiliary electrodes and with a roller-type counterelectrode were employed. But even this arrangement does not fulfill the highest requirements because undesirable effects are produced due to the unavoidable formation of deposit, for example by the dust produced in a preceding cutting operation or by dust particles in the air, these dust particles being deposited on the points and preventing uniform charging.

For de-ionizing dielectric substances, rotary roller electrodes have been disclosed the surfaces of which are studded with discharge points and which cooperate with a smooth roller being the counterelectrode. A disadvantage encountered with this equipment is the fact that the electrons emitted in the direction toward, the roller are easily misdirected due to deposit formation, so that some areas of the surface remain untreated.

Another charging electrode has been proposed in the shape of a roller consisting of a shaft on which a multitude of fixed metallic discs is provided which are arranged parallel with one another in' the axial direction of the shaft.

The choice of discs rather than wires, while avoiding the undesirable oscillations of the corona wires, does not bring about the required reduction of the radius of curvature. Therefore, a considerably more powerful voltage source is required in this case.

All known arrangements have for their object maximum uniformity of the charging or discharging process over the entire surface to be treated. But it has been generally found that, especially with long periods of operation, with comparatively high operating speeds or when processing large surface areas, considerable trouble arose and uniform charging or discharging did not take place, especially in the lower high-tension range. Faults can be remedied only partially and with difficulty, the device having to be removed from a large-sized machine in order to be cleaned or replaced.

Increase of the corona high-tension is an expedient also used, but this requires an accurate variation of the distance between the point of discharge and the treated surface. This variation cannot be effected by an ordinary operator and requires the attention of an expert who, when changing the distance, also has to take into account the saturation limit of the material to be charged. An increase of the corona high-tension by itself is not feasible because it would cause breakdowns.

The present invention provides an apparatus for the electrostatic charging or discharging of the surface of web or sheet-like material, especially the electrostatic charging of photoconductive insulating layers, the apparatus including a charge-absorbing electrode in the form of a rotatable roller with a roughened surface, which roller is partially surrounded by an insulator and is located between pointed corona electrodes in the form of a series of closely spaced needles which are externally protected by a second insulator, the outer extremity of the roller being spaced substantially equally to the points of the needles from the path of a material to be processed by the apparatus. This apparatus satisfies the most stringent requirements and ensures uniform charging or discharging across the entire surface with comparatively low corona voltages, irrespective of running speed and size of the processed surface and notwithstanding long periods of operation.

The apparatus preferably further includes a counterelectrode so arranged that the material can pass between it and the charge-absorbing electrode and adapted to be maintained at the same potential as the charge-absorbing electrode.

The effect achieved by the apparatus of the invention due to the particular arrangement of the needle points in conjunction With the roughened surface of the chargeabsorbing electrode, is that instead of the known coneshaped corona discharges emanating from every needle point, a dense uniform corona discharge is built up in which charge or discharge interruptions are avoided even if an occasional needle point fails to discharge. Furthermore, it has been found that this assembly ensures trouble-free working with a comparatively low corona tension, with constant distance from the processed material and over long periods of operation because the close spacing of the needles reduces the electrical stress on each needle point. A further result of this close spacing is that the unavoidable failure of an occasional needle is compensated by adjacent needles, so that charging or discharging takes place in a uniform manner. If desired the charge-absorbing electrodes also can be provided with a brush which removes the deposit upon it Without interrupting the working operation.

Substances Whose surfaces are to be charged or discharged may be paper, textiles, leather, rubber, plasticcoated metal foils or similar composite materials. A preferred employment for the apparatus of the invention is the electrostatic charging of photoconductive insulating layers.

In view of its intense, uniform action, the apparatus of the invention can be used especially for charging the photoconductive insulating layers in re-enlargement apparatus.

The invention is illustrated by way of example in the accompanying drawings, in which:

FIG. 1 is an elevation of an apparatus of the invention including a counter-electrode,

FIG. 2 is a similar view of a modified embodiment, and

FIG. 3 is a bottom plan view of the apparatus of FIG. 1.

Referring to the drawings, a charge-absorbing electrode 1 is in the form of a solid rotatable roller consisting, for example, of chrome/nickel steel the surface of which is roughened, Roughening can be achieved by grinding, sandblasting, milling or some other form of mechanical treatment and produces a fine surface structure of partly edge-like and partly angular pointed character which aids the trapping or emitting of electrons.

The direction in which the roller rotates is immaterial: rotation may be in the same direction as, or opposite to, the web movement. Preferably, rotation takes place in the same direction as web movement. The two movements may be coupled or may ditfer from one another. An advantageous ratio has been achieved with a rotary velocity of the charge-absorbing electrode about twice the speed of the web.

Corona electrodes 2 extend from a metal profile 9 to which individual, electrically conductive needles 3 are attached at the same level in parallel juxtaposition in comb-like fashion, as seen in FIG. 3. The spacing of the needles or needle points 4 may amount to several needle diameters. More advantageously, the needles are spaced still more closely, however. In a preferred embodiment, the distance between the needles amounts to one needle diameter.

Needle diameters are from approximately 2 mm. to approximately 0.1 mm. Depending upon the material 10 to be charged or discharged, either thicker or thinner needles are used. Thus, when discharging textile surfaces, for example, thick needles will be used. In the case of electrostatically charging photoconductive insulating layers, for which employment the apparatus is particularly suitable, extremely thin needles having diameters of approximately 0. 1 to 1 mm. with very fine points are used. Needle diameters of 0.3 mm, have proved particularly successful.

The material used for the needles is steel, steel alloy or another metal alloy or a noble metal.

The charge-absorbing electrode 1 is situated in the center of the apparatus and is spaced apart from the material 10 to be processed by a perpendicular distance A which is mainly determined by the degree of high tension to be applied and amounts to between 5 and 30 mm. The charge-absorbing electrode 1 is separated from the corona electrodes 2 by an insulator 5. Two corona electrodes 2 are employed for one charge-absorbing electrode 1. It is also possible to arrange several assemblies consecutively, either on a straight or a curved path.

The points 4 of the corona electrodes 2 are equidistant from the charge-absorbing electrode roller 1 and extend parallel to the axis of the latter so that the outer extremity 7 (FIG. 2) of the charge-absorbing electrode 1 is approximately on a level with the needle points 4. Small level difierences are permissible, but care should be taken that the corona discharge takes place only from the needle points 4.

The direction of the needle arrangement of the corona electrode 2 with reference to the web plane 10 and the charge-absorbing electrode 1 can vary in such a way that the needles 3 extend either parallel to the plane of the web 10 and in a direction towards the charge-absorbing electrode 1 or at an angle to the normal to this plane.

Y 4 Advantageously, the corona electrodes 2 are at an angle of 0 to about '60" to the Web plane, as shown in FIGS. 1 and 2. It would instead be possible to position the needle array perpendicular to the web plane and correspondingly to incline the needles in a direction away from the charge-absorbing electrode 1. The selected angular arrangement also depends on whether a fiat construction or a rather more compact shape, for use in a larger machine, is to 'be employed.

The perpendicular distance B between the needle points 4 and the surface of the charge-absorbing electrode 1 depends upon the degree of high tension to be applied and amounts to about 5 to 15 mm. If the high tension is substantially increased, this distance may be larger. With a high tension of 10 kv. the distance is about. 10 mm.

The charge-absorbing electrode 1 may be provided with a cleaning device 12, for example in the shape of a fixed or rotary wiping brush which removes deposit adhering to the roller without interrupting the working operation. The cleaning brush may be provided inside a recess of the insulator S.

The insulator 5 of the charge-absorbing electrode is of U-shaped cross-section, the width of the inner space depending upon the diameter of the charge-absorbing electrode. The charge-absorbing electrode should be enclosed to such an extent that only a minor part of its surface projects beyond the limbs of the U. In a preferred embodiment of the invention, the charge-absorbing electrode 1 projects only a few millimeters, e.g. l to 3 mm., beyond the insulation. The U-limbs are preferably bevelled at their ends.

An insulator 6 outside the corona electrodes 2 shields these electrodes from external contact and may take the form of a housing. The insulating material may consist of a suitable insulating substance of known type, especially a plastic material.

As can be seen from FIG. 2, the insulator 5 may include ducts 11 for the admission of gases or the extraction of gases and/or dust particles. For the purpose of extraction a small degree of suction only is employed whereby the undesirable gases generated during charging or discharging are removed without, however, impairing the movement of the web or sheet. If stronger suction is required, a duplicate arrangement is advantageously used. On the other hand, the introduction of air can improve web transport. When this method is applied, for example, an electrophotographic material becomes pressed against the counter-electrode 8 whereby contact is improved, the distance A is kept constant and steady web transport is achieved.

The position of the ducts 11 in the insulator 5 can be as desired. Advantageously, however, the ducts 11 extend perpendicularly to the web 10 when suction is used and at an acute angle thereto and in the direction of web movement when air is introduced.

The counter-electrode 8 may be a cylinder or a metallic or metallized conveyor belt.

Taking into account the material to be processed and the size and form of the latter, the apparatus of the invention may be equipped for high tensions in the region from 1 to 15 kv. or more.

For the purposes of the treatment of photoconductive insulating layers, high tensions of about 4 to about 12 kv. are suitable.

The apparatus is suitable for the charging or discharging of dielectric materials which travel at a speed of 1 to 50 rn./min., preferably 4 to 20 m./min.

By employing the apparatus of the invention, correct saturation was achieved for zinc oxide paper of DIN A4 size with a running speed of 11 m./min. when applying a high tension of 6000 volts. The needles employed had a diameter of 0.3 mm. and the distance between needles was 0.3 mm. The perpendicular distance (B) between the needle points and the charge-absorbing electrode was 10 mm. and the distance (A) between the charge-absorbing electrode and the paper was 18 mm.

Using an apparatus comprising point electrodes in combination with laterally arranged auxiliary electrodes, the correct saturation for zinc oxide paper was achieved only with the low speed of '6 m./min. and with a high tension of 10,000 volts.

It will be obvious to those skilled in the art that many modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.

What is claimed is:

1. An apparatus for the electrostatic charging or discharging of the surface of web or sheet-like material, especially the electrostatic charging of photoconductive insulating layers, said apparatus including a charge-absorbing electrode in the form of a rotatable roller means with a roughened surface, said roller means being partially surroundedby an insulator and located between pointed corona electrodes in the form of a series of closely spaced needles which are externally protected by a second insulator, the outer extremity of the said roller means being spaced substantially equally to the points of the needles from the path of a material to be processed by the apparatus.

2. An apparatus as claimed in claim 1, further including a counter-electrode so arranged that the said material can pass between it and the charge-absorbing electrode and adapted to be maintained at the same potential as the charge-absorbing electrode.

3. An apparatus as claimed in claim 1, in which the distance between the individual needles amounts to one needle diameter.

4. An apparatus as claimed in claim 1, in which the individual needle diameter is 0.1 to 1 mm.

5. An apparatus as claimed in claim 4, in which the individual needle diameter is 0.3 mm.

6. An apparatus as claimed in claim 1, in which the corona electrodes are at an angle of 0 to to the path of the material. I

7. An apparatus as claimed in claim 1, in which at least one of the said insulators is provided with ducts for the introduction or removal of air.

References Cited UNITED STATES PATENTS 3,581,149 5/1971 Tanaka et a1 317-4 LEE T. HIX, Primary Examiner US. Cl. X.R. 

